Field of the Invention
The invention relates to a method for the hot-dip coating of metal strip, in particular steel strip, in a metallic melting bath, in which method the metal strip to be coated is heated in a continuous furnace and is introduced into the melting bath through a snout which is connected to the continuous furnace and which is immersed into the melting bath.
Description of Related Art
The hot-dip coating of metal strip, in particular steel strip, is a method that has been known for many years for the surface finishing of fine sheet-metal strip in order to protect it against corrosion. FIG. 3 illustrates, in a vertical sectional view, a section of a conventional installation for the hot-dip coating of a metal strip 1. A steel strip (fine sheet-metal strip) which is to be correspondingly finished is, for this purpose, initially cleaned, and subjected to recrystallization annealing, in a continuous furnace 2. Subsequently, the strip 1 is subjected to hot-dip coating by being led through a molten metal bath 3. As coating material for the strip 1, use is made for example of zinc, zinc alloys, pure aluminum or aluminum alloys.
The continuous furnace 2 typically comprises a directly heated preheater and indirectly heated reduction and holding zones, and also downstream cooling zones. At the end of the cooling zone, the furnace 2 is connected via a port (snout) 6 to the melting bath 3. A diverting roller (Pott roller) 7 arranged in the melting bath 3 causes the strip 1 entering the melting bath from the snout 6 to be diverted into a substantially vertical direction. The layer thickness of the metal layer which serves for anti-corrosion protection is normally set by way of stripping jets 5.
As a steel strip 1 passes through the melting bath 3, an alloy layer composed of iron and the coating metal is formed on the surface of the strip. Above this, the metal layer is formed whose composition corresponds to the chemical analysis of the metal melt situated in the melting bath vessel 4.
Depending on the melt composition, the coating has different characteristics, in particular with regard to mechanical and anti-corrosion protection characteristics. Also, the melt composition has an influence on the reliability of a process with regard to surface quality of the coated strip. In practice in the prior art, it is therefore the case that a corresponding composition of the metallic melting bath is selected in a manner dependent on the desired characteristics, that is to say, with a compromise solution, there is always a balancing act between the requirements such as, for example, the mechanical characteristics for the subsequent deformation of the coated fine metal sheet with the avoidance of cracks in the coating or peeling of said coating, on the one hand, and reliable anti-corrosion protection, on the other hand.